1. Field of the Invention
The invention relates to a brake control system. More particularly, the invention relates to a brake control system that controls braking force applied to a wheel by switching a valve interposed in a hydraulic fluid path between being open and closed.
2. Description of the Related Art
In recent years there has been much progress in the development of electronically controlled brake systems that aim to improve running stability and vehicle safety by electronically controlling the braking force applied to each of a plurality of wheels of a vehicle. These electronically controlled brake systems control the wheel cylinder pressure to apply the optimum braking force to each wheel by controlling a plurality of valves interposed in a hydraulic fluid path between being open and closed. One example of such an electronically controlled brake system is the vehicular brake system disclosed in Japanese Patent Application Publication No. 2004-322843 (JP-A-2004-322843), which has a master cutoff valve that is a normally open valve, which is open when the brake is not applied and closed when the brake is applied.
With the vehicular brake system described in JP-A-2004-322843, hydraulic fluid (i.e., brake fluid) supplied to a wheel cylinder is returned to a reservoir tank via a pressure decrease linear valve when the brake is released from an applied state. In this case, if there is a brake fluid leak at a caliper, for example, less brake fluid will return to the reservoir tank. Also, there is some variation in the amount of brake fluid in the reservoir tank when the vehicle leaves the factory and some vehicles may be shipped from the factory with a relatively small amount of brake fluid in the reservoir tank. In addition, a brake system normally includes a mechanism that adjusts the position of the piston as the brake pads wear, and thereby the amount of brake fluid stored in the caliper increases as the pads wear. Further, low temperatures cause brake fluid to contract, reducing its volume. Therefore, even if there is no brake fluid leak, there may be less brake fluid in the reservoir tank. Less brake fluid in the reservoir tank may result in less brake fluid in the master cylinder, which may in turn affect the operability of the brake pedal by the driver.